The present invention relates to blind rivets.
Blind rivets are used in a variety of applications, from securing thin sheets to relatively thick components, and in a variety of materials, ranging from steel to plastics. The blind rivet enables lower cost assembly to be achieved, but also contributes to the strength and integrity of the resulting joint or fixing. For example, if a pair of workpiece sheets to be joined are slightly bowed, the rivet needs to clamp the sheets together.
A blind rivet is generally required to have high shear strength, and to expand into any clearance between the rivet body and a hole in the workpiece into which the rivet is inserted, in order to prevent the sheets of the workpiece from fretting under applied vibrations or oscillating applied loads. This is especially important in the automotive industry.
It is also generally required that the rivets when set should have a single roll-type bulged setting on the blind side of the workpiece, giving an increased tensile resistance to applied loading, while providing an aesthetic appearance.
In addition, it is generally desirable that the mandrel of the rivet break flush in the region of the rivet flange, contributing to an increase in the shear resistance of the joint. In order to achieve these requirements, especially in the case of higher strength blind rivets, the mandrel head is required to be fixed to the tail end of the rivet body on assembly and to remain in place during and after setting, thus contributing to the resistance to moisture ingress through the bore of the rivet body. In addition, by securing the mandrel head in the rivet body, rattling of the mandrel head within the set rivet is avoided.
EP 0677666 discloses a blind rivet having a recess beneath the mandrel head of the rivet. The tail end of the rivet body is crimped into an elliptically shaped recess beneath the mandrel head, and a further indentation is provided near to a flange portion of the rivet body.
GB 402813 shows a blind rivet having a mandrel head having a bevelled surface beneath the head. This rivet suffers from the drawback that it has a relatively narrow grip range.
GB 2231932 shows a blind rivet having a mandrel provided with grooves beneath its head. On assembly of the rivet, the tail of the rivet body is swaged into the grooves to retain the mandrel in place during manufacture. As the mandrel stem is pulled to set the rivet, that part of the rivet body swaged to the mandrel beneath the mandrel head bulges and begins to be forced outwards and out of engagement with the grooves in the mandrel stem. As the bulging process proceeds with setting, and the mandrel continues to move through the rivet body, the mandrel grooves then move into engagement with the internal bulge bore and the mandrel is locked in place. However, this rivet suffers from the drawback that there is often insufficient engagement of the mandrel to the rivet body to prevent movement of the mandrel head relative to the rivet body. In addition, when set in minimum grip thickness, this rivet has the disadvantage of providing a multi-fold rather than a single roll-type setting.
U.S. Pat. No. 6,004,086 discloses a blind rivet in which the mandrel is provided with grooves beneath the mandrel head. This rivet suffers from the drawback that if the rivet body is formed from stronger material such as steel, the higher setting loads required to set the rivet cause the mandrel head to be pulled into the rivet body before setting is complete.
EP 1106845 discloses a blind rivet having a recess beneath the mandrel head, into which the tail end of the rivet body is formed. This rivet suffers from the drawback that the setting load for certain materials such as steel is high and as the maximum load is approached, the end of the rivet body deforms and the internal bore moves away from the recess formed below the mandrel head. This causes a gap to form between the recess and the rivet body, which enables the mandrel head to move relative to the rivet body when the mandrel breaks.
GB 2416575 discloses a blind rivet in which a series of annular grooves are provided beneath the mandrel head. Although this type of rivet provides an adequate grip range, the load required to set the rivet is high compared with other blind rivets, as a result of which the mandrel head tends to loosen under recoil forces as the mandrel breaks at completion of setting of the rivet.
It can therefore be seen that all of the above rivets suffer from the drawback that it is difficult to secure the mandrel head in the rivet body to a sufficient extent during setting of the rivet. Preferred embodiments of the present invention seek to overcome one or more of the above disadvantages of the prior art.